Digital transmission systems that allow a large number of transmitters, each with a plurality of data channels, to share a single band of the frequency spectrum are used in cellular communications and the like. Within each transmitter, each data stream is spread over the frequency band and combined with other orthogonally spread data streams. This combined spread spectrum signal is then scrambled using a code that allows the combined data streams from one transmitter to be separated upon reception from those of other transmitters in the same frequency band. In the following discussion, this final spread and scrambled signal will be referred to as the “chip-rate” analog waveform. The chip rate analog waveform is then up-converted to the desired transmitter frequency.
In a number of applications, it is desirable to measure the characteristics of the chip rate analog waveform from a specific transmitter for diagnostic purposes. Prior art methods for measuring the chip rate analog waveform of a transmitter require that the signal transmitter be isolated from other transmitters, since each additional transmitter appears to be a noise source with respect to the transmitter whose signal is to be measured. As a result, such measurements have typically been made in a laboratory setting in which there are no competing transmitters to interfere with the measurement of the desired transmitter. While such measurements are useful in characterizing a transmitter, they are not applicable to characterization in actual use in the field. Accordingly, it would be advantageous to be able to measure the chip rate analog waveform of a transmitter in the field during actual operation and in the presence of competing transmitters.